The present invention relates, in general, to a unique microstructure and to fabrication techniques for such structures, and more particularly relates to self-aligned nanometer sized probe tips and methods for fabricating such tips. Still more particularly, the present invention relates to techniques for fabricating stacked, aligned integrated tunneling tips for use in sensors and analytical instruments based on movable, self-aligned, finely pointed tip pairs. Such pairs are formed on single crystal, polycrystalline, or amorphous silicon beams and are fully integratable with electronic circuits on a silicon chip.
Scanning-probe microscopes are instruments which provide precise surface measurements through the use of a probe positioned in very close proximity to the surface of interest. The probe scans the surface in a controlled fashion to provide information related to the nature of the surface, including its topography and its chemical and electronic structure. Highly resolved spatial measurements of characteristics such as electron energy states, magnetic field strengths and directions, thermal conductivity and response, and interatomic forces can be made with high resolution at or near atomic levels. Many versions of such devices utilize a movable pointed tip that can be precisely positioned in three dimensions.
One of the principle applications of scanning probe microscopy takes advantage of the spatial selectivity of electron tunneling, using a scanning probe as the aperture of the microscope. Such a device, known as a scanning tunneling microscope (STM) does not use an external source of radiation to illuminate a sample under study, but instead uses electrons already present on the surface of the sample and on a finely-pointed tip mounted on the movable probe. Since the device is not limited by the wavelength of a radiation source, its resolution is based on a near-field "aperture" that is a single atom in diameter at the tip of the scanning probe.
The scanning tunneling microscope operates to position an extremely sharp probe tip within a few atomic diameters of a surface to be studied. The STM provides a bias voltage between the tip and sample so that a flow of electrons occurs, with the electrons tunneling from filled states in the surface to empty states in the tip (or vice versa). The tunneling current is measured at multiple points across the surface of the sample, and since the current varies in accordance with the distance between the tip and the surface, this current can be used to control tip height above the surface so that the tunneling current remains constant. Variations in the control signals for positioning the probe and for maintaining this tip height provide a measure of the surface configuration. In addition, since the tunneling current is also a function of the applied tip-to-sample voltage, variation of the bias voltage over a controlled range permits the STM to provide spectroscopic data about the sample. The problem of creating an extremely sharp probe tip has been solved in the past by, for example, sharpening a fine wire of tungsten, platinum, or platinumiridium by cutting, grinding or etching techniques. The tip is positioned with respect to the surface under study, typically by means of a coarse positioning mechanism which may include a micrometer or similar mechanical mechanism and a fine positioning mechanism which may include the use of piezoelectric material. Such arrangements have enabled the STM tip to be positioned to within fractions of an Angstrom from a surface, thereby enabling the tip to be used to locate small amounts of material, or even individual atoms, in a surface.
Since the magnitude of the tunneling current between a probe tip and a surface is exponentially dependent on the distance between the two, a decrease of one Angstrom in this distance would produce a ten-fold increase in the tunneling current. Such large increases permit the use of a constant-current method of maintaining a constant distance between the scanning tip and the sample surface, and thus provide precise sensing and control of the spatial position of the tip. This precise control is the basis for a large number of tunneling devices and systems in the prior art. Descriptions of scanning tunneling microscopes are found in "Vacuum Tunneling: A New Technique for Microscopy" by Calvin F. Quate, Physics Today, August 1986, pages 26 through 33; in "Scanned-Probe Microscopes" by H. Kumar Wickramasinghe, Scientific American, Oct. 1989, pages 98 to 105; and in U.S. Pat. No. 4,912,822 to Zdeblick et al, issued Apr. 3, 1990.
A continuing problem in such devices is the provision of accurate sensing of the distance between a tip and a surface to be sensed, and in providing rapid and accurate feedback to move the tip to prevent contact between the tip and the surface. This requires a low mass mounting for the tip so that even minute feedback signals will be sufficient to effect motion of the tip to allow rapid scanning of a surface. Further, to be useful in microelectronics, the device needs to be not only small, but should be of a material which enables it to be integral with the electronic control circuits which regulate its motion and which detect the tunneling current. Preferably, the scanning-tunneling device is of silicon so that it can be integrated on a silicon chip.
Prior devices which have relied on piezoelectric transducers for tracking surface contours and for scanning the tip have required that suitable piezoelectric devices be mounted on the tip or on its support. This has required the additional fabrication step of providing piezoelectric materials on the tip support, making fabrication of the device more complex and more difficult. Furthermore, this additional material affects the mass of the scanning-tunneling device and thus its resonant frequency, thereby limiting control of the motion of the device, and thus limiting its range of operation.
Prior scanning-tunneling devices have mounted the tip adjacent to a surface to be scanned, where the surface is mounted for motion in the X-Y plane, with the tip being movable in the vertical, or Z-direction to prevent contact between the tip and the planar surface. In order to provide a high degree of control of such motion, it has been found necessary to construct such devices so that they are relatively stiff. However, such stiffness not only places limits on the motion of the tip, but also reduces its response to forces such as vibration or acceleration so that the device is less capable of measuring these parameters with high sensitivity. Furthermore, the proximity of the tip to a horizontally extending planar surface does not provide a clearly defined reference point for the detection of relative motion, thus further limiting the use of these devices as sensors.